1. Field of the Invention
The present invention generally relates to a vehicle drive train control system. More specifically, the invention relates to a vehicle drive train control system configured to disengage a traction control clutch.
2. Description of Related Art
Currently, automotive OEMs are introducing brake based vehicle stability control systems that use vehicle sensors, such as, wheel speed, yaw rate and steering angle sensors to detect when the vehicle is traveling in a direction different from the direction intended by the driver. Such systems use a steering wheel angle sensor to detect the intended direction of the vehicle. A yaw or yaw rate sensor and the existing wheel speed sensors from the ABS are used to detect the actual direction and speed of the vehicle. By comparing the intended direction and the actual direction, the electronic controls will apply braking torque to one or more wheels to bring the vehicle back to the direction intended.
For example, the vehicle stability control system will, typically, apply the outside front brake during an oversteer condition. The vehicle stability control system will apply the inside rear brake when an understeer is detected. In addition, the vehicle stability control system may automatically reduce the throttle to minimize the chances for loss of control. However, vehicle stability control systems have limitations with regard to traditional vehicle stability and traction control systems.
Currently, vehicle stability control systems are not installed on vehicles with locking rear differentials or locking transfer cases. A locking differential locks the right and left wheels together to improve traction, preventing one wheel from slipping. If the vehicle stability traction control system applies braking action to a wheel on the one side, both wheels would be effectively braked through the locking differential engagement. Similarly, a transfer case that locks the front and rear axles together would limit the effectiveness of the vehicle stability control system. Locking the front and rear axles together prevents the independent application of braking torque to only one wheel. Even if the clutch locking the differential or transfer case is turned off immediately upon the onset of the vehicle stability control system, the differential or transfer clutch mechanism may not be disengaged quickly or effectively enough to prevent interference with the vehicle stability control system.
Under some circumstances, the application of brake torque to certain wheels will lock the differential or transfer case clutches into engagement even more. For example, if a roller clutch locking differential is locked into engagement when the vehicle is decelerating hard while turning in an understeer condition, application of the inside rear brake will keep the roller clutch stuck in the engaged position even if the engagement signal is interrupted. This stuck condition only makes the control problem worse. Similarly, during heavy deceleration while turning left, the roller clutch will be engaged so that an outer race attached to the right wheel is trying to overrun an inner race attached to the left wheel, because of the larger turning radius of the outside wheel. Therefore, the heavy deceleration locks the roller clutch into one side of the engagement wedge, effectively locking the right and left wheels together, despite their different turning radil. For clarity, the inside will be used to refer to the side of the vehicle toward which the vehicle is turning. Similarly, the outside will be used to refer to the side of the vehicle away from which the vehicle is turning. Therefore, if the vehicle is making a right turn, the inside would be the right side of the vehicle, typically the passenger side in the United States.
In an understeer condition, the vehicle stability control system will apply brake torque to the inside rear wheel to correct the vehicle. The inside wheel in a left turn is the left wheel that is attached to the inner race. Again the braking torque locks the clutch even harder into engagement thereby defeating the vehicle stability control system.
Other locking clutch systems such as dog clutches or pin lockers would also be locked into engagement because of the inherent friction between the clutch components. For example, when the normal braking torque is fed across the mating gear type components of a dog clutch, the friction between the teeth prevents the gears from being easily separated thereby keeping the clutch locked in the engaged position. Similarly, even a small braking torque across a pin locker locking differential will keep the pins pressed against a receiver socket, preventing them from being pushed back out of their engaged positions, thereby keeping the differential locked. Further, clutch pack limited slip differentials rely on a cam mechanism that must be unwound by reverse rotation of the two sides of the differential, inhibiting the release of the clutch mechanism when residual braking torque is still present across the clutch. Accordingly, automotive OEMs decline to offer vehicle stability control systems on vehicles with locking differentials or locking transfer cases.
Vehicle stability control systems can provide an improvement with regard to traction by applying the brake to the slipping wheels, thereby forcing drive torque in the non-slipping wheels. However, such activity results in excessive wear of brake pads and rotors, and can even cause overheating of the braking system. Therefore, it would be desirable to use a vehicle stability control system that can operate in a vehicle with a locking transfer case or locking differential.
In view of the above, it is apparent there is a need for an improved vehicle stability control system.